On the Scaling of Zinc Oxide Thin-Film Transistors

Abstract

Metal-oxide thin-film transistors (TFTs) are quickly emerging as the industry standard for state-of-the-art flat panel displays and large-area sensing systems. Yet, due to the poor performance of non-crystalline semiconductor materials processed at low temperatures (<650ºC), TFTs still remain a bottleneck in large-area electronics (LAE), often limiting performance and consuming significant energy. This work aims to evaluate the scalability of TFTs with LAE-compatible processing for improving device performance. This is done by studying the effect of scaling transistor channel length and source/drain-to-gate overlap down to 0.5 µm in staggered bottom-gate ZnO TFTs. At the device level, contact resistance and thermal stability are identified as the dominant drawbacks in scaled devices, whereas short-channel effects are overcome by use of a thin gate dielectric (40 nm Al2O3 in this work). Despite these non-idealities, the high-frequency performance of ZnO TFTs significantly improves from scaling. We demonstrate TFTs with cut-off frequency ft = 800 MHz and maximum power gain frequency fmax = 2.7 GHz, representing a 100x improvement over a 5-µm minimum feature size technology.

The capabilities of scaled ZnO TFTs are also demonstrated at the circuits and systems levels. A TFT-based HF RFID-reader array which can be integrated into large surfaces is demonstrated, achieving a read range of 2.5 cm, showcasing the capabilities of TFT technology for power transfer in the MHz regime. Second, the first-ever gigahertz Metal-Oxide TFT-based LC oscillators are demonstrated, operating at 1.25 GHz at VDD=6.65 V with a 0.5 mm2 footprint. At such frequencies, LAE can be used for RF communication. In this regard, a 1 GHz fully-LAE compatible 3-element phased-array transmitter is demonstrated. LAE can thus prove to be a unique technology for monolithically integrated dense, large-aperture phased-arrays. For both systems, it was simultaneous optimization at the device, circuits, and systems level which enabled such achievements.